1. Field of the Invention
The present invention relates to a physical quantity sensor made by microfabricating a silicon substrate, and in particular, to a physical quantity sensor having high hermeticity and high electrical stability.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication No. 2007-266320 discloses an invention related to a sensor module. The sensor module includes a sensor substrate, and an electrode-forming substrate and a circuit-forming substrate, respectively facing upper and lower surfaces of the sensor substrate. The electrode-forming substrate and the circuit-forming substrate are bonded to the sensor substrate through metal sealing layers. Gaps between these substrates are sealed by Au—Au bonding, which is room temperature bonding.
A via hole extends through each of the sensor substrate and the electrode-forming substrate from an upper surface to a lower surface thereof. According to Japanese Unexamined Patent Application Publication No. 2007-266320, the via holes are electrically insulated from the substrates.
The technology for sealing gaps between substrates with a metal, which is described in Japanese Unexamined Patent Application Publication No. 2007-266320, is also disclosed in International Publication No. WO2003/001217.
As described in Japanese Unexamined Patent Application Publication No. 2007-266320, a physical quantity sensor includes a plurality of silicon substrates that are stacked with insulation layers therebetween, and each of the silicon substrates is in an electrically floating state (is not electrically connected to any other parts). Therefore, there is a problem in that it is difficult to improve the electrical stability of the physical quantity sensor, because the physical quantity sensor has a large parasitic capacity and is likely to generate noise.
A physical quantity sensor includes a movable portion for detecting a change in physical quantity and a frame portion surrounding the movable portion. The movable portion occupies a large space in the middle of the physical quantity sensor. Gaps between upper and lower surfaces of the frame portion and substrates having insulation layers thereon are sealed (see Japanese Unexamined Patent Application Publication No. 2007-266320). Therefore, a structure for reducing parasitic capacity and improving electrical stability needs to be disposed in a very limited region outside of the movable portion. Moreover, it is necessary to avoid negative influence on metal sealing layers for sealing the gaps between the substrates, which are disposed near this region.